Method for preventing formation of a slug flow regime of a gas-liquid mixture in a non-linear wellbore or pipeline

ABSTRACT

A method for preventing formation of a slug flow regime of a gas-liquid mixture in a non-linear wellbore or a pipeline comprises determining at least one most probable place of fluid slugs development in the wellbore or the pipeline by mathematical simulation based on expected values of the gas-liquid mixture flow and known geometry of the wellbore or the pipeline and mounting a device, in the determined place of fluid slug development, that converts the stratified gas-liquid mixture flow into a dispersed flow.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Russian Application No. 2016129941 filed Jul. 21, 2016, which is incorporated herein by reference in its entirety.

BACKGROUND

The disclosure relates to methods for preventing formation of slugs of fluid blocking a pipe cross-section when transporting a gas-liquid mixture in wellbores and pipelines during production of hydrocarbons in the oil and gas industry.

In cases where a wellbore or a pipeline is not straight and in addition to the horizontal sections it contains ascending, descending and vertical segments, the so-called slug flow regime can be established. In this regime, intermittent single-phase portions of gas and liquid (slugs) are transported in the pipe. The high-speed slug flow regime is dangerous for surface equipment. It also leads to pressure oscillations at the bottom-hole of a wellbore, which in turn can lead to undesirable geomechanical effects, such as damage to a near-wellbore part of a stratum and deterioration of its conductivity, extensive solid-phase migration to the wellbore, and proppant backflow, especially in case of stimulation by a multistage hydraulic fracturing of the stratum. In the horizontal part of the pipe, the stratified flow regime is usually established, in which gas moves over a layer of heavier liquid (oil or oil with mixture of water). When the pipeline deviates from the horizontal level, a pipe lumen in the knees can be blocked periodically by the liquid accumulated in lower regions of the pipeline. In the case where the horizontal section becomes vertical, a fluid slug is also developed. When the mixture enters the vertical section, rapid segregation takes place, in which the liquid blocks the entrance to the vertical section, thus preventing the free passage of incoming gas. As a result of development of a fluid slug blocking the pipeline, the gas is accumulated behind it, in which the pressure rises with time. In the course of time, the pressure of the accumulated gas becomes sufficient to push through a portion of the accumulated fluid, thus establishing the non-stationary flow regime with periodic splashes of large portions of the liquid phase.

Various methods for controlling slugging in a gas-liquid mixture flow are known from the prior art. Thus, U.S. Pat. No. 6,041,803 A describes a device and a method for preventing formation of slug flow regime in an ascending segment in which the inlet stratified flow is converted into a non-stratified regime (annular or bubble). The apparatus includes a convergent nozzle and a divergent diffuser.

Application US 20090301729 A1 discloses a device for controlling the slugging in a multi-phase flow, being a pipe having a specific structure of sections of 1 to 30 feet long, inclined at an angle of 5-90 degrees to the horizon.

U.S. Pat. No. 6,716,268 B2 relates to control of development of slugs in a riser by controlling gas pressure at the bottom of the riser. For this purpose, a separation tank is used, which includes a valve to control gas velocity in a pipeline. If the monitored pressure exceeds some empirically determined value, the valve opens and changes the gas velocity, which prevents the appearance of plugs.

The article “Experimental investigation of terrain slugging development, evolution and potential for mitigation” of Brasjen, B. J., et. al., 16th International Conference on Multiphase Production Technology, The BHR Group, 2013, describes a set of devices to be mounted in circumhorizontal inclined pipes for dissipation of fluid plugs. In the experiments, the authors observed the decrease in pressure fluctuation up to 16%.

The known methods and devices are mainly intended to be used only at the entrances to risers and do not allow ensuring the reliable prevention of slug flow regime formation, since they consider neither the expected flow rate of the gas-liquid mixture nor the geometry of the pipeline.

SUMMARY

The disclosure ensures reliable prevention of slugs development during a gas-liquid mixture flowing in non-linear wellbores and pipelines due to consideration of an expected flow rate of the gas-liquid mixture and a geometry of a wellbore or a pipeline when choosing the mounting location of the device for preventing slugs.

The disclosed method for preventing formation of a slug flow regime of a gas-liquid mixture in a non-linear wellbore or a pipeline comprises determining at least one most probable place of fluid slugs development in a wellbore or a pipeline by mathematical simulation based on expected values of the gas-liquid mixture flow and the known geometry of the wellbore or the pipeline and mounting a device, in the determined place of fluid slug development, that converts the stratified gas-liquid mixture flow into a dispersed flow.

Different types of devices, for example, vortex-type devices, twisted-tube bundle devices, mixer-type devices, rotating brush type devices, can be used to convert the stratified gas-liquid mixture flow into the dispersed flow.

BRIEF DESCRIPTION OF DRAWINGS

The disclosure is explained by the drawings, where FIG. 1 shows a diagram of a gas-liquid mixture flow with a fluid slug developed in a wellbore having inclined and vertical sections;

FIG. 2 shows a diagram of the gas-liquid mixture flow in the same wellbore with the devices mounted therein, that provide conversion of the stratified flow into the dispersed bubble flow;

FIG. 3 shows an example of a device for conversion of the slug flow regime into a dispersed one, configured as a bundle of tubes,

FIG. 4 shows an example of a device for conversion of the slug flow regime into the dispersed one, configured as a rotating brush;

FIG. 5 shows an example of a pipeline geometry,

FIG. 6 shows a slope of the pipeline of FIG. 5, depending on the coordinate of the pipe,

FIG. 7 shows distribution of oil volume fraction across the pipeline at the moment of appearance of fluid slugs, obtained as a result of mathematical simulation.

DETAILED DESCRIPTION

The disclosed method is aimed at preventing formation of a slug flow regime in inclined and vertical sections of a wellbore or a pipeline, in those places where such regime is most probable upon results of mathematical simulation of the gas-liquid flow in a wellbore or a pipeline. In mathematical simulation, a geometry of the wellbore or the pipeline is used as obtained, for example, from a drilling log for the case of a wellbore, or directly measured where possible. For the expected flow rates, the flow regimes of the gas-liquid mixture in the wellbore or the pipeline containing, in addition to the horizontal section, inclined and vertical parts, are determined based on the numerical simulation, and at least one most probable place of development of fluid slugs is detected. Devices that convert the stratified flow of the gas-liquid mixture into the dispersed bubble flow are mounted in the determined most probable places of development of fluid slugs, which significantly increases the segregation time and significantly reduces the period between the fluid slugs and thus alleviates negative consequences of the slug flow regime.

The geometric configuration of a wellbore such as that shown in FIG. 1 leads to the formation of a slug flow regime in vertical and inclined sections due to gravitational segregation of the gas-liquid flow. A gas-liquid mixture enters a wellbore 1 from the side 2 and exits from the side 3. Here, a liquid phase 4 can move as a layer under a gas phase 5 and develop slugs in the zones 6 a and 6 b due to the influence of gravity and deviations of the pipeline from the horizontal position. At bottom points of the inclined sections, heavy fluid accumulates and blocks the lumen of the pipe 6 a, then the gas-liquid flow from the circumhorizontal section enters the vertical part where, due to the gravity action, rapid segregation occurs and a heavy liquid blocks a lumen of the pipe, thereby preventing free passage of the gas. As a result, pressure in the blocked gas volume rises and the fluid slug is pushed upward. Such splashes produce high-frequency pressure oscillations, which in turn can lead to undesirable geomechanical damage to the near-wellbore area, reduction in conductivity of the crack and reduction in hydrocarbon production from the stratum.

The risk of geomechanical damage to the near-wellbore area directly depends on the rate of change in pressure, i.e. from the derivative of the pressure over time (the higher its value, the higher the risk of damage). Thus, the decrease in frequency of pressure oscillations helps to reduce the risk of damage to the stratum.

To prevent high-frequency pressure oscillations, it is proposed to increase segregation time as far as possible in the regions where slugs are most likely to appear. For this purpose, it is proposed to convert the stratified flow into the dispersed flow by means of special devices. The stratified flow, passing through such devices, will be converted into a bubble or gas-droplet flow (depending on the volume fractions of the phases). For a dispersed flow, the segregation time is significantly higher, which will either completely prevent the development of fluid slugs or significantly reduce the rate of their development. FIG. 2 shows a diagram of gas-liquid mixture flow in a wellbore having inclined and vertical sections where the stratified flow 8 is converted by means of a special device 7 a into the dispersed bubble flow 9, which in turn can be segregated into the stratified one and then re-converted into the dispersed flow 10 by means of the device 7 b.

To convert the stratified flow into the dispersed one, the devices of various types can be used, for example, of the vortex type (http://www.chengfluid.com/), in the form of a twisted-tube bundle (FIG. 3), various variants of mixers (http://Www.stamixco-usa.com/plug-flow-reactors), in the form of a rotating brush (FIG. 4), etc.

The device shown schematically in FIG. 3, is a bundle of twisted tubes (not less than 2). Being mounted in the stratified gas-liquid flow, this device redirects the gas phase from the upper part of the flow to the lower one, and the liquid phase—in the opposite way, which leads to phase mixing and formation of a dispersed mixture. The length of such device shall exceed the diameter of the pipeline. The device can be made of various materials, for example, plastic or metal.

The device of FIG. 4 is a rotating brush for mixing the stratified gas and liquid phases. For effective mixing, the length of such device shall exceed the diameter of the pipeline. The device can be made of various materials, for example, plastic or metal. The exact position of the device is determined on the basis of mathematical numerical simulation of the gas-liquid flow in a pipeline of a given configuration.

The method can be implemented as follows.

Based on the well-known pipeline geometry derived from direct measurements or basing on drilling log data and typical flow rate of the phases, one can determine the possibility of establishment of the slug flow regime, as well as the exact place of the slug development, for which purpose, mathematical numerical simulation is used. Simulation can be based on solving non-steady-state equations of a multi-fluid model or a drift model derived from the laws of conservation of mass and pulse of continuum mechanics. The details of these methods and the features of the numerical solution of the determining equations are presented, for example, in the work (Theuveny B. C. et. al. Integrated approach to simulation of near-wellbore and wellbore cleanup//SPE Annual Technical Conference and Exhibition. —Society of Petroleum Engineers, 2013).

FIGS. 5-7 show an example of numerical simulation of the oil and gas flow in a pipeline. FIG. 5 is a general view of the pipeline geometry, FIG. 6 shows the slope of the pipeline depending on the coordinate of the pipe defining the side-view in FIG. 5a . Numerical simulation was carried out for constant oil and gas flow rates specified at the pipeline inlet and presented in Table 1. The outlet of the pipeline is considered to be open into a region with constant atmospheric pressure (see Table 1).

TABLE 1 The gas flow rate at the pipeline inlet, m/s 0.14 The oil flow rate at the pipeline inlet, m/s 0.027 Pipeline outlet pressure, atm. 1.0 Coordinates of slug development points, m S₁ = 150.5 S₂ = 758.9

FIG. 7 shows distribution of oil volume fraction across the pipeline at the moment of appearance of fluid slugs, obtained as a result of mathematical simulation. The points S₁ and S₂ in FIG. 7 denote the sites where the slugs will appear and before which it is necessary to mount a device for converting the stratified flow into the dispersed one. The calculated exact coordinates of these points are given in Table 1. 

1. A method for preventing formation of a slug flow regime of a gas-liquid mixture in a non-linear wellbore or pipeline, the method comprising: determining at least one most probable place of fluid slugs formation in the non-linear wellbore or pipeline by mathematical simulation based on expected values of the gas-liquid mixture flow and known geometry of the wellbore or the pipeline, and mounting a device, in the determined place of fluid slug development, that converts a stratified gas-liquid mixture flow into a dispersed flow.
 2. The method of claim 1, wherein a vortex-type device is used as the device that converts the stratified gas-liquid mixture flow into the dispersed flow.
 3. The method of claim 1, wherein a twisted-tube bundle device is used as the device that converts the stratified gas-liquid mixture flow into the dispersed flow.
 4. The method of claim 1, wherein a mixer-type device is used as the device that converts the stratified gas-liquid mixture flow into the dispersed flow.
 5. The method of claim 1, wherein a rotating brush type device is used as the device that converts the stratified gas-liquid mixture flow into the dispersed flow. 